Hitherto, a water-absorbing resin has been used as one of the constituent materials of sanitary cotton, disposable diapers, or other hygienic materials that absorb body fluids. As such a water-absorbing resin, for example, there are a hydrolysate of a starch-acrylonitrile graft polymer, a neutralized product of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylic acid ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer, a crosslinked body thereof, and a partially neutralized poly(meth)acrylic acid crosslinked body. Among them, a water-absorbing resin formed from a partially neutralized polyacrylic acid (salt) crosslinked body is often used from the viewpoint of the absorbing properties. These all have a crosslinked structure and are insoluble in water.
The properties desired for such a water-absorbing resin include a high fluid retention capacity, a high water absorption speed, an excellent suction power to suck up water from a base material, high liquid permeability, and the like. Among these, liquid permeability of a water-absorbing resin is understood as the ability to transport the liquid added within the particles or between the particles and to three-dimensionally distribute the liquid in the swollen state thereof. In the case of particulate water-absorbing resin, the transport by a capillary action through the gap between the gel particles of the swollen water-absorbing resin is dominant. Hitherto, in a water-absorbing resin that cannot maintain the capillary voids by the gel alone under a load due to the lack of stability of the gel, the mutual separation between the particles is secured by holding these materials in a fiber matrix. However, in the structure of diapers of the new generation, a fibrous material for supporting the liquid transport by the water-absorbing resin is used only in a small amount or not used at all. Hence, the water-absorbing resin used therein is required to have sufficiently high stability in the swollen state. A water-absorbing resin is required to have a swollen gel elastic modulus in order to achieve high stability in the swollen state.
For the purpose of improving various kinds of absorption properties such as a swollen gel elastic modulus of the water-absorbing resin, an operation to form a crosslinked structure in the vicinity of the surface of the water-absorbing resin by using a crosslinking agent having a plurality of functional groups capable of reacting with a carboxyl group present in the water-absorbing resin and thus to increase the surface crosslinking density of the water-absorbing resin (surface crosslinking) has been hitherto conducted (Modern Superabsorbent Polymer Technology, 1998, pp. 55-60 and pp. 97-103). In addition, when a partially neutralized polyacrylic acid internally crosslinked body (base polymer) is synthesized before the surface crosslinking as well, it is conducted to improve the swollen gel elastic modulus by a technique to add a chain transfer agent at the time of polymerization (JP 2005-111474 A) or a technique to increase the amount of a radical initiator to be used at the time of polymerization (JP2009-531467 W). The mechanism of the improvement of the swollen gel elastic modulus by these techniques is considered to be as follows. That is, it is possible to lower the weight average molecular weight of the main chain polymer in the water-absorbing resin by adding a chain transfer agent or increasing the amount of a radical initiator at the time of polymerization. Due to this, entangled crosslinkings of the main chain polymer decrease. The entangled crosslinkings suppress the swelling of gel. Hence, in a case when a chain transfer agent is added or the amount of a radical initiator is increased at the time of polymerization, it is required to compensate the decrease in entangled crosslinkings with chemical crosslinking by increasing the amount of an internal crosslinking agent to be used in order to obtain a water-absorbing resin having an equal equilibrium absorption capacity as a water-absorbing resin obtained by normal polymerization. As a result, in the water-absorbing resin obtained by these techniques, the proportion of the entangled crosslinkings in the internal crosslinked structure becomes lower and the proportion of chemical crosslinking becomes higher as compared to a water-absorbing resin obtained by normal polymerization. It is considered to be important that the proportion of chemical crosslinking is higher in this way, in order to achieve a high swollen gel elastic modulus.
However, a great difference in weight average molecular weight of the main chain between the water-absorbing resin obtained by these techniques and a normal water-absorbing resin, and the amount of a chemical crosslinking agent for achieving an equal equilibrium absorption capacity is also not significantly different. Hence, it is considered that a great number of entangled crosslinkings are still present even in the crosslinked structure of the water-absorbing resin obtained by these techniques, and it is thought that there is room for significant improvement.
Note that, here, it is considered that the achievement of a high swollen gel elastic modulus cannot be expected by only decreasing the proportion of entangled crosslinkings by simply further decreasing the weight average molecular weight of the main chain. This is because it is considered that it is required not only to decrease the weight average molecular weight of the main chain but also to narrow the molecular weight distribution of the main chain at the same time for the reasons as described below in order to achieve a high swollen gel elastic modulus. That is, in the case of a water-absorbing resin having a small weight average molecular weight of the main chain and a wide molecular weight distribution, a great number of significantly short main chains are present in the crosslinked structure thereof. The number of crosslinking points contained in such a significantly short main chain is considered probabilistically significantly small. In a case when the crosslinking points are fewer, the proportion of the length of a dangling chain to the total chain length is increased. The dangling chain here refers to a terminal portion of the main chain that is not sandwiched between a crosslinking point and another crosslinking point. In an extreme case, the main chain that has only one crosslinking point is a dangling chain in its entirety. Since a dangling chain does not contribute to the swollen gel elastic modulus, it is preferable to decrease the dangling chains and increase the main chains that are effective in the elastic modulus in order to improve the swollen gel elastic modulus. Accordingly, it is considered that it is required to prevent the generation of significantly short main chains by narrowing the molecular weight distribution of the main chain in order to suppress the generation of dangling chains.
From the matters described above, it is considered that the entangled crosslinkings or dangling chains present in the crosslinked structure can be significantly decreased and it can be expected to have a high swollen gel elastic modulus if it is possible to obtain a water-absorbing resin having a small weight average molecular weight of the main chain, a narrow molecular weight distribution of the main chain, and a uniform network structure with a uniform mesh size.
Hitherto, studies on the synthesis of a gel having a uniform network structure with a uniform mesh size have been carried out, and for example, a technique to synthesize a polyacrylic acid ester crosslinked body having a uniform network structure by crosslinking the terminal of a straight-chain polymer having a functional group at both terminals with a star-shaped low molecule is disclosed in J. A. Johnson at al., J. Am. Chem. Soc., 2006, 128, pp. 6564-6565. In addition, whereas the disclosure of J. A. Johnson et al., J. Am. Chem. Soc., 2006, 128, pp. 6564-6565 mostly relates to a polyacrylic acid ester crosslinked body, it is described that “this polyacrylic acid ester crosslinked body can be converted to a polyacrylic acid crosslinked body.”
In addition, with regard to the “synthesis of a nonionic hydrophilic gel having a uniform network structure”, there are numerous known literatures relating to the synthesis of a polyethylene glycol crosslinked body through a reaction of two kinds of star-shaped polymers and the evaluation on physical properties thereof (for example, T. Sakai et al., Macromolecules, 2008, 41, pp. 5379-5384).
Note that, in the prior arts relating to the “synthesis of a gel having a uniform network structure” such as J. A. Johnson et al., J. Am. Chem. Soc., 2006, 128, pp. 6564-6565 and T. Sakai et al., Macromolecules, 2008, 41, pp. 5379-5384, it is not disclosed at all to form an ionic network structure by reacting two or more kinds of star-shaped polymers.